Apparatus for the spectroscopic analysis of materials



W. L. HOFF Aug. 12, 1941.

APPARATUS FOR THE SPECTROSCOPIC ANALYSIS OF MATERIALS Filed July 23, 1940 UV VE/V 70/? W L HOFF a) anama 5y the analysis results Patented Aug. 12, 1941 APPARATUS FOR THE SPECTROSCOPIC ANALYSIS OF MATERIALS Wilbur L. Hoff, Hollywood, 111., assignor to West-v ern Electric Company, N. Y., a corporation of Incorporated, New York, New .York

Application July 23, 1940, Serial No. 346,963

11 Claims.

This invention relates to apparatus for the spectroscopic analysis of materials, and more particularly to electrodes employed in such apparatus.

In many manufacturing operations it is desirable to determine the exact composition of the material at various stages of the manufacturing process, and it is often important to make the determination rapidly as well as accurately. For example, in metal compounding operations, process control is often based upon periodical analysis of the melt and the effectiveness of the control is usually dependent upon the speed with which are available.

Spectroscopic methods of analysis are preferred for this purpose because they are inherently rapid in comparison with most chemical procedures. One type of spectoscopic apparatus which is used generally coi'iiprises a pair of electrodes, one of which supports the specimen under analysis, a current source for producing an arc between the electrodes, and an optical system for separating the radiant energy from the arc into its component rays. The separated rays are projected onto a photographic plate or a screen in a pattern which indicates the composition of the specimen in accordancewith well known practices. It has been customary in this apparatus to support the specimen on one electrode and to employ a second electrode centered on thespecimen. This construction has been used successfully with a variety of material but it is-not satisfactory for some types of analysis. For example, attempts to use this prior structure for quantitative analysis of certain lead base compositions have not been successful because the results were not sufflciently accurate or repetitive.

An object of this invention is the provision of an improved electrode structure for' use in spectrographs and similar apparatus.

An electrode structure provided in accordance with one embodiment of the invention comprises a vertically mounted lower electrode having a cylindrical body with a cavity in its top surface to receive the specimen, and a vertically mounted upper electrode having a cylindrical body and a tapered lower end portion formed thereon; The tapered portion is formed cooperatively by two fiat side surfaces and a fiat bottom surface all positioned at predetermined angles to the body axis and converging at a common point on the body periphery. In mounting the electrodes, this point is aligned with a point on the lower elec-' trode top surface outside of the cavity.

-- men under Following is a more complete description of the invention, taken in conjunction with the appended drawing, in which Fig. l is a perspective view showing the es sential members of a spectrograph incorporating an electrode structure embodying certain features of the invention;

Fig. 2 is an enlarged side elevation of the electrode structure shown in Fig. I viewed in the direction of the arrows 22;

Fig. 3 is an elevation of Fig. 2, viewed in direction of the arrows 3-3;

Figs. 4 to 7 illustrate the steps in the manufacture of the upper electrode shown in the other figures, wherein Fig. 4 is an elevation of the electrode at one stage of its manufacture;

Fig. 5 is a view similar to Fig, 4 showing the electrode at a subsequent stage of its manufacture;

Fig. 6 is a side view of Fig. 5, and

Fig. 7 is a view similar to Fig. 6 showing the contour of the completed electrode.

As stated above, this electrode structure is suitable for use in an otherwise conventional spectrograph, such as of Fig. 1.

is illustrated in a portion In this type of apparatus, the speciinvestigation is supported on a lower electrode III which is connected by a wire II to one side of a suitable direct current power source (not shown). Positioned above the lower electrode is an upper electrode l2 which is connected to the other side of the power source through a wire l3. The application of current to the electrodes produces an are I 4 between them and the arc rays impinge against I5 which is positioned adjacent to the electrodes and base very small slit l6 therein. This slit is only a few thousandths of an inch in size and it passes a thin beam I! of are light, usually through a collimating lens (not shown), to a prism l8 which refracts rays of different wave length to a different degree and thus effects a separation, or analysis, of the component rays of the arc. The refracted rays l9 pass to a sensitized photographic plate 20 and are reproduced thereon in a pattern 2| which shows, by comparison with standards, the composition of the specimen. The general structure and operation of the shield, lens, prism and plate are conventional, and this invention struction and arrangement of the two electrodes.

The lower electrodedll, which is mounted vertically, is formed from a rod, preferably cylindrical in shape, of spectrographically pure cara flat plate or shield.

is directed to the conin extent to the cavity diameter, around the cavity. The cavity depth depends upon the size of the specimen, but a fairly shallow cavity gives the best results.

The upper electrode is made from a cylindrical rod 25 of spectrographically pure carbon or graphite (Fig. 4), and has especially configured end portion, which is conveniently formed in the series of process steps illustrated in Figs. 4 to 7. In the first operation in the manufacture of this electrode, one end portion of the rod is cut to a wedge or a V-shape to form two wedge faces 26 which are equal in area .and meet in a straight line representing the true diameter of the cylindrical rod. The oblique angle 21 between the two wedge faces, or the wedge angle, should be about 30,providing an angle around ,l5 between each wedge face and the rod axis. These specific values were found to give the best results with a rod around three-eighths inches in diameter, but for other rod sizes the wedge angle may vary between 20 and 40, and the angle between each wedge face and the rod axis may vary accordingly between and 20". I

In the next operation, the wedge portion is bevelled along the juncture of the wedge faces to form a flat tapering bottom surface 281 on the rod. As shown in Fig. 'I, this surface extends across the rod, with its lower end starting from the point where the two wedge faces and the rod periphery meet, In the beveling operation, equal portions of the two wedge faces are cut away with the bottom surface 28 intersecting each of the wedge faces at the same angle and the remaining portions of the wedge form side end faces on the rod. The bevel angle 29, between the bottom surface and the axis of the rod, or between this surface and the rod periphery at their juncture, should be around but bevel angles between 35 and 55 can be used satisfactorily.

The completed upper electrode thus has a tapered end portion formed cooperatively by the rod periphery, the two wedge faces and the bevel surface, all converging to provide a point at the rod periphery. The bevel surface. which approximates a sector in shape (Fig. 3), forms the fiat bottom surface, and the remaining portions of the wedge, having the shape of elliptical segments, provide fiat sidefaces. Although this same contour can be produced in various ways, it is provided most conveniently and economically by the above described procedure.

The completed upper electrode is mounted in the spectrograph in a vertical position with one of the wedge side faces facing the shield 15 and the bottom electrode surface lying in a plane at right angles to the plane of the shield. This arrangement is shown in Fig. 2 which is the view of the electrodes as seen through the slit in the shield.

In the electrode mounting. the two electrodes are offset. Preferably, and especially where the upper and lower electrodes are made from rods of equal diameter, the point on the upper electrode, formed by convergence of the side and bottom surfaces and the periphery, is aligned with the outer periphery of the lower electrode, as shown in Fig. 2, with the outer surfaces of the two electrodes approximately in line and the electrodes offset. However, this arrangement can be modified, particularly where the two electrodes are of different size or an unusually shaped specimen cavity is employed, and in general the pointed end of the upper electrode should be aligned with a point on the upper In the operation of the spectrograph, the

specimen or sample, either as solid material or in the form of an acid solution, is placed in the lower electrode recess. The current, at a fairly high voltage such as 220 volts D. C., is then applied to the electrodes from the power source to produce an arc between the electrodes. The are radiation is projected against the shi ld, which passes a narrow beam to the prism where the rays are separated on the basis of wave length and then recorded on the photographic plate, all as previously described.

With this apparatus, specimens that are not normally adapted to spectrographic methods can be analyzed rapidly and accurately. Due to the construction of the electrodes, complete excitation of the specimen occurs spontaneously with the introduction of current to the electrodes. During excitation of the specimen, cathode wandering, which is usually troublesome in processes of this type, is avoided. The configurations and positions of the end and side surfaces of the upper electrode minimize electrode vapor condensation, which is one source of this trouble and confines unavoidable condensation to locations outside the sphere of arc influence. Consequent- 1y, excitation of the specimen proceeds at a steady rate and constant intensity with the result that a faithful and complete spectral line reproduction, suitable for accurate quantitative analysis of the specimen, can be accomplished with a small sample in a few seconds.

It will be apparent that modifications and adaptations of the specific structures and procedures described herein are feasible, and it is to be understood that the invention is limited only by the scope of the appended claims.

What is claimed is:

1. In an apparatus for the spectroscopic analysis of material, a lower electrode having a cavity in its top portion for containing the material,

' and an upper electrode spaced from the lower electrode, said upper electrode having a cylindrical body with a tapered lower end portion terminating in a point at the periphery of the cylindrical body.

2. In an apparatus for the spectroscopic analysis of material. a lower electrode having a cavity in. its top surface for supporting the material, and an upper electrode having a cylindrical body and a tapered lower end portion terminating at the periphery of the cylindrical body, said upper electrode being mounted with its tapered portion aligned with a point on the top surface of the lower electrode outside of the cavity.

. 3. In an apparatus for the spectroscopic analysis of material, a vertical lower electrode having a cylindrical body for supporting the material, and an upper electrode spaced from the lower ,electrode, said upper electrode comprising a cylindrical body, and a tapered lower end portion terminating at the periphery of the cylindrical body, and said upper electrode being mounted vertically with the lower end of tion aligned with the periphery of the lower electrode.

4. In an apparatus for the spectroscopic analysis of material, a vertically mounted positive electrode of cylindrical shape having a cavity in its top surface for supporting the material, and a negative electrode vertically mounted above the positive'electrode, said negative electrode comprising a. cylindricalbody, and a taperedlower end portion terminating in a point at the periph-- cry of the cylindrical body, the negative electrode being positioned with its point aligned with the periphery of the positive electrode and its body offset from the positive electrode.

5. In an apparatus for the spectroscopic analysis of material, a vertically mounted cylindrical lower electrode having a cylindrical cavity positioned centrally in its top surface for supportingthe material, the diameter of the cavity being about one-third the diameter of the electrode, and an upper electrode spaced from the lower electrode, said upper ele'ctrode comprising a cylindrical'body, and a tapered lower end portion aligned with a point on the top surface of the lower electrode outside of the cavity.

6. In an apparatus for the spectroscopic analysis .of material, a vertically mounted lower electrode having a cylindrical body with a material supporting cavity in its top surface, and a vertically mounted upper electrode spaced from the lower electrode, said upper electrode comprising lower electrode, said upper electrode comprising a cylindrical body, a pair of flat faces on the .ower end portion of said body, each of said faces saving the shape of an elliptical segment and extending upwardly from a common point on the Jody periphery, and a flat tapering bottom surits tapered porsis of material, a vertically side surfaces Join the body face in the form of the point where the side surfaces join the body periphery.

8. In an apparatus for the spectroscopicanalythe lower end portion of said body each extending upwardly from a common point on the body periphery at an angle between 10 and 20 with the axis of the body, and a flat bottom surface extending across the body from the point where the side faces Join the body periphery at an angle between 35 and with the body axis.

9. In an apparatus for the spectroscopic analymounted lower electrode having a cylindrical body with a material supporting cavity in its top surface, and a vertically mounted upper electrode spaced from the lower electrode, said upper electrode comprising a cylindrical body, a pair of flat side faces on the lower end portion of said body each extending upwardly from a common point .on the body periphery at an angle between 10=and 20 with the axis of the body,'and a flat bottom surface on the .body intersecting the two faces at an angle between 35 and 55 the body periphery, cooperatively forming in alignment with a top surface outside cavity.

10. In an apparatus for the spectroscopic analysis of material, a vertically mounted positive electrode having a cylindrical body with a material supporting cavity in its top surface, and a vertically mounted negative electrode spaced faces'and bottom surface a point which is positioned point on the lower electrode of the material supporting above the lower electrode, said upper electrode comprising a cylindrical body, a pair of flat side faces on the lower end portion of said body, each of said faces extending upwardly from a common point on the body periphery at an angle around 15 to the axis of the body, and a flat bottom surface betweenthe two faces extending across the cylindrical body from the point where the periphery at an angle of about 45 to the body axis.

11. In an apparatus for the spectroscopic analysis of material, an

ysis.

WILBUR L. HQFF.

a sector between the faces "and extending across the cylindrical body from with the body .axis, 

